The present invention relates to a small mobile antenna in a mobile satellite communication system. The mobile satellite communication system can provide a high quality communication service in a wide area. A communication service for ships is now available all over the world by using the INMALSAT system. Mobile communication systems for aircraft, and/or land mobile stations have also now been developed. In a mobile satellite communication system, a small antenna which has half sphere coverage does not need to track a desired satellite, and is considered promising for making an antenna small. Further, a circularly polarized radio wave is used for a mobile satellite communication system, and a mobile antenna with a wide angle, and excellent axis-ratio characteristics has been required. With this in mind, a quadrifilar helix antenna which has four coils is considered to be one of the candidates for a small mobile antenna. A prior quadrifilar helix antenna, for example, is shown in "Resonant Quadrafilar Helix" by C. C. Kilgus in IEEE Trans. vol.AP-17, May 1969.
FIG. 7 shows a structure of a prior quadrifilar helix antenna. In the figure, the numeral 41 is a feed circuit, 42 through 45 are feed lines, 46 through 49 are helix conductors. The helix conductors 47, 48 and 49 are fed with the phase differences 90.degree., 180.degree. and 270.degree., respectively, in comparison with that of the helix conductor 46, and the antenna radiates circularly polarized waves. The shape of the antenna is defined by its pitch distance, the number of helix turns and the radius of the helix conductors. One example of each of those parameters for achieving an almost half sphere beam are 1 .lambda. of pitch the distance, 0.5 turn, and 0.1 .lambda. of the radius of helix conductors, where .lambda. is the wavelength to be used.
FIG. 8 shows the radiation characteristics of the prior quadrifilar helix antenna having said parameters. In FIG. 8, .theta. is the angle between a observation point and a helix axis; a solid line and a dotted line show the radiation pattern for a co-circular polarization, and that for the anti-circular polarization, respectively.
The prior quadrifilar helix antenna has a wide beam, and excellent axis-ratio characteristics in a wide area as shown in FIG. 8.
However, a prior quadrifilar helix antenna has the disadvantages that the value of the parameters for the desired characteristics are severely restricted, and it is impossible to provide a smaller-sized antenna.
Further, in a mobile satellite communication system which includes a ship and/or an aircraft, a mobile antenna receives not only a direct wave from a satellite, but also reflected waves by the sea-surface. The direct wave and reflected waves interfere with each other, and the receive level is subject to fading which is called a multipath fading due to sea-surface reflection (denoted by "multipath fading" hereafter).
In a mobile satellite communication system, a power margin is provided so that communication is possible with a defined percent of the time even under the decreased level resulting from the multipath fading. When the power margin is large, a satellite must transmit with high power. The wider an antenna beam and the lower an elevation angle of a satellite are, the larger the effect of multipath fading due to sea-surface reflection. Therefore, it is desirous that the mobile antenna is not affected by multipath fading.
By the way, when a circularly polarized wave is reflected by the sea-surface, the reflected wave has an elliptical polarization whose major axis is almost parallel to the sea-surface. Therefore, from the view point of the rejection of reflected waves, it is preferable that the polarization characteristics of the mobile antenna in the direction of the reflected waves is orthogonal to those characteristics of the reflected waves. In other words, it is preferable that the major axis of an elliptical polarization is directed as vertical as possible.
However, a prior quadrifilar helix antenna has the elliptical polarization in which the major axis is directed in an almost horizontal direction, and therefore, it tends to be affected by the multipath fading. This is explained in accordance with FIG. 9, which shows the polarization characteristics of the sea-surface reflection waves with the elevation angle of 5 degrees, and those of the antenna's sea-surface reflection direction (5 degrees under horizon) of a prior quadrifilar helix antenna. In the figure, the numeral 61 is the sea-surface, 62 is the polarization characteristic of the sea-surface reflection waves, and 63 is polarization characteristics of a prior quadrifilar helix antenna.
It should be noted in FIG. 9 that a prior quadrifilar helix antenna has the elliptical polarization whose major axis is essentially parallel to the sea-surface. Therefore, a prior antenna receives a significant amount of the sea-surface reflection waves, and is subsequently affected by the multipath fading.